This invention relates to apparatus for applying power to a heater filament of a glow plug projecting into a combustion chamber in a diesel engine. The glow plug is heated by applying a source of power to the heater filament contained therein. The source can be, for example, a conventional vehicle battery which is also used to energize a starter for the engine. The heater glow plug facilitates diesel engine starting by raising the temperature of air in the combustion chamber from ambient temperature to a temperature sufficiently high to start the engine. Therefore, an operator of the engine must wait a relatively substantial period of time before the glow plugs in the engine have been sufficiently heated to facilitate diesel engine starting.
Because the battery is a source of power for the starter and other equipment as well as the heater filament, the voltage level of the battery fluctuates. A decreasing battery voltage lengthens the period of time the operator must wait before the glow plugs have been sufficiently heated. One method of controlling filament temperature is to employ direct temperature feedback from the filament. However, such a method requires more complex and expensive electronic components as well as a direct connection to the filament during the engine operation. Another problem involves deenergizing the filament within some time period after the engine has started to prevent overheating of the filament.